DRO tuning screw strange behaviour

[Terminator3]

As i know, frequency of DRO must increase if distance between screw and DR decreases.
But I am observing decrease, and then some increase of frequency while turning screw clockwise (distance decreases). Original DRO enclosure was without tuning screw, i added hole and aluminium screw on top of DR.

 

[biff44]

When you lower a tuning screw to the top of a dielectric puck, you are capacitively loading it. The same way as if you squirted some glue on the top of the dielectric puck lowers the frequency, moving the screw closer will lower the resonant frequency.

 

[Terminator3]

Thank for your reply! Yesterday i was reading http://www.4timing.com/technical_considerations.htm, and in "4. Mechanical Consideration" i can't understand this phrase:

Finally, the fine tuning and adjustment of the DRO will be set through a tuning screw that will increase the DR’s resonant frequency as it closes the electrical field above the puck. This should provide as much 80 MHz of tuning range. However, it is important to notice that tuning the frequency with a tuning screw is achieved at the cost of reduction in both unloaded Q and temperature stability. This worsen of temperature stability is due to the increasing slope of the tuning curve as the metal plate gets closer to the DR surface.

Here: http://www.satelliteguys.us/xen/threads/dtv-lnb-mod-ku-conversion-part-one.30837/

I lowered the LO 200 MHz by adding additional ceramic to the puck. Modifying the puck will avoid the loading down effect on the oscillator caused by attempting to lower the frequency by adjusting the screw too close to the puck.

So here we lower frequency by decreasing spacing between DRO and screw.
Tuning screw too close = lower frequency.

But here:tmo.jpl.nasa.gov/progress_report/42-162/162C.pdf

Since the substrate dimensions are held constant, the change in the
microwave resonance is determined by the change in the cavity height from the alumina substrate to
the metal tuner. As the spacing between the DRO puck and the metal tuner is decreased, the resonant
frequency increases and the circuit Q drops, thus increasing electronic tuning bandwidth.

So here we increase frequency by decreasing spacing between DRO and screw.
Tuning screw too close = higher frequency.

I do not get something here....

Also just found this:
**broken link removed**

22. The DRO puck adjustment screw needs to have a larger diameter at the end nearest the puck itself. I soldered / glued a washer onto the end of the bolt, it could also be achieved by inverting the screw and cutting a slot in the other end to adjust it (electrical connection is not necessary as the adjustment is a mechanical alteration of the “size” of the cavity created by the puck itself – the closer the screw is to the puck the HIGHER the frequency as the size of the cavity is then smaller).

Tuning screw too close = higher frequency...

 

[vfone]

In most of the applications the DRO is inside of a cavity and the screw mechanism is used to alter the cavity oscillation resonant frequency.
When a metal wall of the resonant cavity is moved toward the interior, the resonant frequency will decrease if the stored energy is mainly in electric field.
When the stored energy close to the cavity walls is mostly magnetic, as in the case of shielded dielectric resonators, the resonant frequency will increase when the metal wall move toward the interior.

 

[Terminator3]

Any good online information about DRO enclosure design? I found rule of thumb: enclosure must be more than three pucks in width and height: **broken link removed**
And some info on this page: http://www.trans-techinc.com/products_detail.asp?ID=11&Name=Dielectric-Resonators

 

[jiripolivka]

I can see that the discutees never saw a real DRO. Tuning by a conductive round screw head really should increase the DRO frequency when the head closes to the dielectric puck.But when it goes farther away, the frequency may increase with distance.
Make sure the cavity does not resonate close to DRO frequency, otherwise you have two coupled resonators in action.